Overripe fruit on the ground, slightly swollen, a little soft, fragrant… and slightly alcoholic. This is what our ancestors of the hominoid lineage, today represented by great apes and humans, often found in the tropical forests of Africa. And they loved this little taste of ethanol.
According to the “Drunken Monkey” hypothesis, formulated by Robert Dudley in the year 2000, ethanol consumption among humans does not come from modern cultural degeneration, but from ancient evolutionary pressure. Millions of years before man invented agriculture and fermentation, our fruit-eating ancestors would have already been fans of naturally fermented fruits. And the passage from the tops of the trees to the ground with the gleaning of fruits that had fallen to the ground would have accelerated this evolution.
A change that changes everything
A study published in 2014 reports that about 10 million years ago, a mutation in the ADH4 (alcohol dehydrogenase) enzyme allowed our ancestors to metabolize ethanol with forty times more efficiency. This adaptation occurred precisely in our ancestors of the hominoid lineage (common ancestor of humans, chimpanzees and gorillas but not in orangutans).
For what? Because they lived high up in the trees, where the fruits do not ferment as much as on the ground. The ancestors of chimpanzees and gorillas willingly went down to glean fallen fruits, which were more likely to have fermented under the natural action of yeast. Having access to many fermented fruits while being able to break down alcohol then became an evolutionary advantage, with this mutation becoming a “gain of function” making it easier to find high-calorie food.
Ethanol is widely present in nature, mainly through the fermentation of fruit sugars by yeast.
In the same way that the mutation of ADH allowed primates to adapt to improve their diet, humans have also adapted more recently with mutations in the other enzyme, ALDH, which degrades acetaldehyde (toxic degradation product inducing an intolerance reaction to alcohol with facial redness (a “flush”), tachycardia, nausea) into acetate (non-toxic). This reaction of intolerance to alcohol is supposed to protect us from the consumption of alcohol, a molecule which with its equally toxic metabolite can cause severe damage to our health.
Ethanol as an olfactory treasure map
The ethanol emitted by fermented fruits is not only felt up close. It spreads over distance and probably served as an olfactory signal for primates to locate energy resources rich in sugar.
Ethanol is widely present in nature, mainly through the fermentation of fruit sugars by yeast. One study reported the alcohol concentrations of different fruits that had not yet fallen to the ground but some of which were overripe. The concentration varies according to species and environments: traces (0.02 to 0.9%) have been measured in temperate and subtropical fruits such as mountain ash, sycamore fig or date palm, while in humid tropical areas, particularly favorable to fermentation, much higher levels (up to 10% in certain palm fruits in Panama) have been observed in overripe fruits.
Although most fruits have low levels (less than 0.2% on average), their repeated consumption can represent a significant source of ethanol for fruit-eating animals. This natural production of alcohol is part of a complex relationship between plants, yeasts and animals: fruits offer sugars, yeasts colonize and ferment, and attracted animals help disperse both seeds and yeast spores, suggesting a form of “ecological mutualism”.
Our attraction to alcohol is not a modern anomaly. It’s an evolutionary legacy, a bug from the rainforest days, when ethanol meant calories and survival.
Many anecdotes circulate about animals being “drunk” after consuming fermented fruits… Elephants and baboons in Africa with marula, or even a moose in Sweden stuck in a tree after eating fermented apples. However, these stories rarely remain scientifically validated: neither the ethanol content of fruits nor the presence of alcohol or its metabolites in animals have been measured. On the other hand, we know that certain mammals, such as green monkeys introduced to the Caribbean, do not hesitate to steal and consume alcoholic fruit cocktails left unattended by tourists on the beaches of Saint Kitts and Nevis.
A study reports that chimpanzees from Bossou in Guinea use rolled up leaves as “sponges” to drink fermented palm sap containing up to 6% ethanol. And it’s not drinking in secret: they drink in groups, passing the leaves around, almost like a community aperitif. Enough to revisit our ideas about alcohol and social bonds. The activity of gleaning fermented and alcoholic fruits on the ground then becomes a social motivation, already! We didn’t invent anything.
These days, alcohol is flowing freely, but our enzymes haven’t kept up
Evolution has given us a liver capable of breaking down about 7 grams of ethanol per hour. It’s not much compared to modern cocktails. In the past, the volume of ethanol ingested was naturally limited by what was contained in… a monkey’s stomach full of fruit. Today, we can drink tens of grams in a few sips of highly alcoholic drinks. A shot (a “baby”) of whiskey or spirits at 40 degrees and only 3 centiliters contains 10 grams of alcohol!
It is therefore not surprising that we must face the problem of excessive alcohol consumption and alcohol addiction which have serious consequences on health and our societies. We talk about “mismatch” and “evolutionary hangovers”. Mutations in our genes encoding ADH and ALDH have not yet allowed our species to cope with the harmful consequences of excessive alcohol consumption.
In conclusion, our instinct to drink is old, but the risks are new.
Our attraction to alcohol is not a modern anomaly. It’s an evolutionary legacy, a bug from the rainforest days, when ethanol meant calories and survival. But in a world where alcohol is concentrated, accessible, omnipresent… what was an advantage has become a risk factor for our health and a priority public health issue.
